The mouse cell line, 3T3-4E, is unable to synthesize NAD from nicotinic acid, and 3T3-4E cells stop growth in medium containing nicotinic acid as the only precursor to NAD. In contrast, human-3T3 hybrids grow continuously in this medium. We will analyze chromosome segregation from human-3T3 hybrids to identify the human chromosomes that confer upon the hybrids the ability to utilize nicotinic acid. Using the conventional biochemical and cellular techniques of electrophoresis of enzymes, paper chromatography of intracellular metabolites and autoradiography, we will determine which human chromosomes contain the loci for the NAD biosynthetic enzymes, nicotinic acid phosphoribosyltransferase, NAD synthetase, and NAD pyrophosphorylase. We will also determine whether the inability of 3T3-4E to utilize nicotinic acid results from normal developmental regulation, a regulatory mutation or a defect in the transport or phosphoribosylation of nicotinic acid. In a separate project we will undertake extensive cytological studies in the attempt to determine the mechanism underlying human chromosome loss from newly-formed human mouse hybrid cells. Among the various measurements we will make are the kinetics of human chromosome loss, the number and positions of centrioles during early cell divisions in hybrid cells, the fate of 3H-thymidine labeled human chromosomes and the positions of human chromosomes within hybrid interphase nuclei and metaphase plates.